1. Field of the Invention
The present invention relates to an optical amplification repeating system having at least one repeater disposed in the middle of an optical transmission line connecting two terminal stations and, more particularly, to such a system that allows the signal-to-noise ratio in the repeater to be measured in an in-service state of the system.
2. Description of the Related Art
Such an art is known that achieves optical amplification of a signal light by supplying the signal light and pumping light with predetermined wavelengths to a doped fiber, i.e., a fiber doped with a rare earth element such as erbium. Recently, there have been developed optical amplifier repeaters on the principle of such optical amplification. In an optical amplification repeating system with such repeaters inserted therein in multiple stages, it is effective to measure the signal-to-noise ratio (SNR) in each of the repeaters in locating a point of trouble. However, there has been developed no such SNR measuring circuit that can be easily mounted in a repeater. Therefore, it has been strongly desired to make it possible to measure the SNR in each optical amplifying repeater in an in-service state, i.e., while the system is in operation.